1. Field of the Invention
The present invention relates to a fuel combustion control apparatus for an engine mounted on an automotive vehicle, for instance, and more specifically to a fuel combustion control apparatus by which ignition timings (advance angles) can be determined on the basis of two, regular and highoctane, maps. Here, the fuel octane rating (number) is an index indicative of fuel antiknocking properties.
2. Description of the Prior Art
In general, when a spark ignition engine is driven by burning fuel at a high compression ratio or at advanced ignition timing, there exist problems in that the engine is overheated; the engine performance is decreased; and further the engine is damaged. The antiknocking property of fuel can be represented by octane rating. The octane ratings of gasoline now on sale are 83 octane in the case of regular gasoline and 93 octane in the case of super gasoline (referred to as highoctane gasoline). From the standpoints of engine thermal efficiency, highoctane gasoline is advantageous because it is possible to increase combustion speed as fast as possible and advance ignition timing as early as possible as long as the engine is operating without producing knocking.
Therefore, there are automotive vehicles on which an engine designed for highoctane gasoline is mounted (referred to as highoctane vehicles). However, if regular gasoline is used erroneously or unavoidably, there exists a problem in that a sufficient engine output power determined on the basis of highoctane gasoline is not generated or knocking is often produced and therefore the engine characteristics are deteriorated. Further, when highoctane gasoline is used for an automotive vehicle on which an engine designed for regular gasoline is mounted (referred to as regular vehicles), it is impossible to effectively utilize the antiknocking characteristics of the highoctane gasoline.
To overcome this problem, usually two ignition timing control maps are previously prepared for both regular and highoctane gasoline so that an appropriate map can be selected automatically according to the fuel to be used. Here, a control map represents appropriate relationship between ignition timings (advance angles) and engine speeds.
An example of fuel combustion control apparatus for an engine as described above is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 60-212673. In this apparatus, immediately after an engine has been started, the ignition timing is first controlled in accordance with a predetermined regular map; when the engine speed increases into a predetermined range, the ignition timing advance angle is incremented at a predetermined constant rate; when the incremented advance angle exceeds a reference map switching line without producing engine knocking, the fuel is determined as highoctane gasoline and therefore the regular map is switched to the highoctane map.
Further, while the highoctane map is being used, when knocking is produced, the advance angle is decremented, and when the decremented advance angle goes below the maximum highoctane map retard angle line, the highoctane map is switched to the regular map.
In the above-mentioned prior-art fuel combustion control apparatus for an engine, however, since two switching lines, that is, the reference map switching line along which the regular map is switched to the highoctane map and the maximum highoctane map retard angle line along which the highoctane map is switched to the regular map are provided, there exists a problem in that hunting occurs between the regular map and the highoctane map when the ignition timing advance angle is incremented in the regular map to between the reference map switching line and the maximum highoctane map retard angle line. This problem will occur under the following conditions, for instance: when the regular map is switched to the highoctane map within the range where the maximum highoctane map retard angles are larger than the reference map switching angles; since the advance angle is smaller than the maximum highoctane map retard angle at the succeeding control operation, the highoctane map is switched to the regular map again.